cmd/go: explicitly reject module paths "go" and "toolchain"
The module paths "go" and "toolchain" are reserved for the dependency on
the go and toolchain versions. Check for those paths in go mod init to
create modules, go mod edit, and in the module loader and return an
error when attempting to use those paths for a work module. Trying to
init or load a work module with a go.mod that specifies the module path
"go" panics since Go 1.21 (when the toolchain switching logic and the
implicit dependencies on the "go" module was introduced), and this
change returns a proper error instead of panicking.
Fixes #74784
Change-Id: I10e712f8fddbea63edaeb37e14c6d783722e623f
Reviewed-on: https://go-review.googlesource.com/c/go/+/691515 Reviewed-by: Ian Alexander <jitsu@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Matloob <matloob@google.com>